Silicas

Description

The invention relates to silanised, structurally modified, pyrogenically produced silicas, a process for the production thereof and their use.

Silanised silicas are used as thickeners, such as e.g. for water-thinnable lacquers and resins, such as e.g. epoxy resins.

From EP 0 672 731 B1, silanised, pyrogenically produced silicas are known, which are characterised in that the pyrogenically produced silicas are treated with a compound from the group (RO)₃SiC_nH_2n+1, wherein n=10 to 18 and R=short-chained alkyl radicals. For example, the pyrogenically produced silicas have been treated with the compound (CH₃O)₃SiC₁₆H₃₃ (hexadecyltrimethoxysilane) or with the compound (CH₃O)₃SiC₁₈H₃₇ (octadecyltrimethoxysilane).

The production of the silanised, pyrogenically produced silicas takes place in that the pyrogenically produced silicas are placed in a mixer, the silicas are sprayed, optionally first with water and then with the compound from the group (RO)₃SiC_nH_2n+1 while mixing intensively, mixed for a further 15 to 30 minutes and then tempered at a temperature of 100 to 160° C. for a period of 1 to 3 hours.

The invention provides silanised, structurally modified, pyrogenically produced silicas characterised by groups fixed on the surface, the groups being alkylsilyl (SiC_nH_2n+1, with n=2-18), preferably octylsilyl and/or hexadecylsilyl.

The silica according to the invention can have the following physico-chemical characteristics:

BET-surface area m2/g: 25-400

• Average size of the primary particles nm: 5-50
• pH value: 3-10
• Carbon content %: 0.1-25
• DBP value %: The DBP value is at least 10% lower than the DBP value of the corresponding silanised, non-structurally modified silica. With very marked structural modification, the structure can be broken down in such a way that the DBP value can no longer be determined.

A silica produced by a high-temperature hydrolysis route from SiCl₄+H₂ and O₂ can be used as the pyrogenically produced silica.

In particular, a silica produced by high temperature hydrolysis having the following physico-chemical characteristics can be used:

	TABLE 1
	AEROSIL	AEROSIL	AEROSIL	AEROSIL	AEROSIL	AEROSIL	AEROSIL	AEROSIL
	90	130	150	200	300	380	OX 50	TT 600

Behaviour in respect of water	hydrophilic
Appearance	loose white powder

BET surface area1)	m2/g	90 ± 15	130 ± 25	150 ± 15	200 ± 25	300 ± 30	380 ± 30	50 ± 15	200 ± 50
Average size of the	nm	20	16	14	12	7	7	40	40
primary particles
Tamped density2)
standard material	g/l	ca. 80	ca. 50	ca. 50	ca. 50	ca. 50	ca. 50	ca. 130	ca. 60
compacted material	g/l	—	ca. 120	ca. 120	ca. 120	ca. 120	ca. 120	—	—
(additive “V”)
Loss on drying3)		<1.0	<1.5	<0.59)	<1.5	<1.5	<1.5	<1.5	<2.5
(2 hours at 1000° C.)	%
on leaving supplier's
works
Loss on ignition4)7)	%	<1	<1	<1	<2	<2	<2.5	<1	<2.5
(2 hours at 1000° C.)
pH value5) (in 4%		3.6-4.5	3.6-4.3	3.6-4.3	3.6-4.3	3.6-4.3	3.6-4.3	3.8-4.8	3.6-4.5
aqueous dispersion)
SiO28)	%	>99.8	>99.8	>99.8	>99.8	>99.8	>99.8	>99.8	>99.8
Al2O38)	%	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.08	<0.05
Fe2O38)	%	<0.003	<0.003	<0.003	<0.003	<0.003	<0.003	<0.01	<0.003
TiO28)	%	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03
HCl8)9)	%	<0.025	<0.025	<0.025	<0.025	<0.025	<0.025	<0.025	<0.025
Sieving residue6)	%	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.2	<0.05
(acc. to Mocker,
45 μm)
1)based on DIN 66131
2)based on DIN ISO 787/XI, JIS K 5101/18 (not sieved)
3)based on DIN ISO 787/II, ASTM D 280, JIS K 5101/21
4)based on DIN 55 921, ASTM D 1208, JIS K 5101/23
5)based on DIN ISO 787/IX, ASTM D 1208, JIS K 5101/24
6)based on DIN ISO 787/XVIII, JIS K 5101/20
7)based on the substance dried for 2 hours at 105° C.
8)based on the substance ignited for 2 hours at 1000° C.
9)HCl content is a component of the loss on ignition

Pyrogenic silicas of this type are known. They are described, inter alia, in:

• Winnacker-Küchler, Chemische Technologie, volume 3 (1983), 4^th edition, page 77 and
• Ullmanns Encyklopädie der technischen Chemie, 4^th edition (1982), volume 21, page 462.

The pyrogenically produced silicas are treated with a compound from the group (RO)₃SiC_nH_2n+1, wherein n=2 to 18 and R=alkyl, such as e.g. methyl, ethyl or similar.

In particular, the following compounds can be used:

• Silane I (CH₃O)₃SiC₁₆H₃₃ (hexadecyltrimethoxysilane)
• Silane II (CH₃O)₃SiC₈H₁₇ (octyltrimethoxysilane)

The silicas according to the invention can be produced in that the pyrogenically produced silicas are placed in a mixer, the silicas are sprayed, optionally first with water and then with the compound (organosilane) from the group (RO)₃SiC_nH_2n+1 while mixing intensively, mixed for a further 15 to 30 minutes and then tempered at a temperature of 100 to 160° C. for a period of 1 to 3 hours, structurally modified and/or optionally post-ground. A further tempering can optionally take place after the structural modification and/or post-grinding.

The structural modification can take place e.g. with a ball mill or a continuously operating ball mill. The post-grinding can take place e.g. using an air-jet mill or pin mill. The tempering can take place batchwise, e.g. in a drying cupboard, or continuously, e.g. in a fluidised bed. The tempering can take place under protective gas, e.g. nitrogen.

The water used can be acidified with an acid, e.g. hydrochloric acid, down to a pH value of 7 to 1.

The organosilane used can be dissolved in a solvent, such as e.g. ethanol.

The tempering can be performed in a protective gas atmosphere, such as e.g. under nitrogen.

The pyrogenically produced silicas according to the invention silanised with silane I have the physico-chemical characteristics listed in Table 2 before structural modification:

TABLE 2
Educt	A 90	A 130	A 150	A 200	A 300	A 380	OX 50	TT 600
Average size of the	20	16	14	12	7	7	40	40
primary particles
[nm]
BET surface area	40-90	60-130	75-150	100-200	150-300	200-380	20-50	100-250
[m2/g]
Tamped density	40-140	40-140	40-140	40-140	40-140	40-140	40-140	40-140
[g/l]
Loss on drying [%]	<2	<2	<2	<2	<2	<2	<2	<2
Loss on ignition	0.1-10	0.1-10	0.1-10	0.5-15	0.5-20	0.5-25	0.1-10	0.1-20
[%]
C content [%]	0.1-10	0.1-10	0.1-10	0.5-15	0.5-20	0.1-25	0.1-10	0.5-20
pH value	3.5-5.5	3.5-5.5	3.5-5.5	3.5-5.5	3.5-5.5	3.5-5.5	3.5-5.5	3.5-5.5

The silanised, structurally modified, pyrogenically produced silicas according to the invention can be used to improve scratch resistance in lacquers.

EXAMPLES

The pyrogenically produced silicas used have the physico-chemical characteristics listed in Table 1.

As organosilanes, the following compound with the general formula (RO)₃SiC_nH_2n+1 is used:

• (Silane I) (CH₃O)₃SiC₁₆H₃₃

The silica is placed in a mixer and sprayed first with water and then with organosilane, mixing intensively.

When the spraying is complete, stirring is continued for a further 15 to 30 minutes and then the mixture is tempered for 1 to 3 hours at 100 to 160° C. The tempering can also take place under protective gas, e.g. nitrogen.

The individual reaction conditions can be taken from Table 3.

The physico-chemical characteristics of the silanised silicas obtained are listed in Table 4.

TABLE 3
							Tem-
			Silane	Water	Ethanol	Tem-	pering
Ex-			quantity	quantity	quantity	pering	temper-
am-			(g/100 g	(g/100 g	(g/100 g	period	ature
ple	Aerosil	Silane	Aerosil)	Aerosil)	Aerosil)	(h)	(° C.)

1	A 300	Silane I	1	0	9	2	120
2	A 200	Silane I	2.5	0	0	2	140
3	A 200	Silane I	20	5	0	2	140
4	A 200	Silane I	10	2.5	0	2	140
5	A 200	Silane I	5	1.25	0	2	140
6	A 200	Silane I	2.5	1.25	0	2	140

TABLE 4
		Tamped		Surface	Loss on	Loss on
		density	C content	area	drying	ignition
Example	pH value	(g/l)	(%)	(m2/g)	(%)	(%)

1	4.3	50	1.3	253	0.4	1.8
2	4.4	49	1.7	176	0.3	2.5
3	4.6	68	10.1	116	0.6	12.7
4	4.5	72	5.7	144	0.6	7.1
5	4.7	52	2.6	167	0.6	3.4
6	4.5	51	1.9	171	0.7	2.5

Production and Physico-Chemical Properties of the Silicas According to the Invention
Production of the Silicas According to the Invention:

The silicas, which can be produced as described in EP 0 672 731, are then structurally modified by mechanical action and possibly post-ground in a mill. A tempering can possibly take place after the structural modification and/or post-grinding.

The structural modification can take place e.g. with a ball mill or a continuously operating ball mill. The post-grinding can take place e.g. using an air-jet mill or pin mill. The tempering can take place batchwise, e.g. in a drying cupboard, or continuously, e.g. in a fluidised bed. The tempering can take place under protective gas, e.g. nitrogen.

TABLE 5
Overview of the production of the comparative silicas and
the silicas according to the invention (Examples)

			Post-
			grinding
			after	Tempering
	Surface-fixed	Structural	structural	after
Designation	group	modification	modification	post-grinding
Comparative	Hexadecylsilyl	No	—	—
silica 1
Comparative	Octylsilyl	No	—	—
silica 2
Silicas 1	Hexadecylsilyl	Yes	No	No
Silicas 2	Octylsilyl	Yes	Yes	No
Silicas 3	Hexadecylsilyl	Yes	Yes	Yes
Silicas 4	Octylsilyl	Yes	No	Yes
Silicas 5	Octylsilyl	Yes	Yes	No
Silicas 6	Hexadecylsilyl	Yes	Yes	No
Silicas 7	Hexadecylsilyl	Yes	Yes	No
Silicas 8	Hexadecylsilyl	Yes	No	No
Silicas 9	Octylsilyl	Yes	Yes	No
Silicas 10	Octylsilyl	Yes	No	No
Silicas 11	Octylsilyl	Yes	Yes	No
Silicas 12	Octylsilyl	Yes	No	No

TABLE 6
Physico-chemical data of the silicas according to the invention (Examples)
and the comparative silicas

	Tamped		Loss on			DBP	BET specific
	density	Loss on	ignition			adsorption	surface area
Designation	[g/l]	drying [%]	[%]	pH value	C content [%]	[%]	[m2/g]

Comparative silica 1	57	0.5	1.8	4.6	1.2	302	195
Comparative silica 2	51	0.6	6.8	5.3	5.4	263	175
Silicas 1	137	0.7	1.9	4.9	1.3	217	193
Silicas 2	112	0.7	7.0	5.8	5.5	145	175
Silicas 3	118	0.7	2.3	5.1	1.3	228	176
Silicas 4	163	0.9	6.7	5.3	5.4	134	176
Silicas 5	114	0.5	7.1	6.0	5.4	142	175
Silicas 6	113	1.3	2.2	5.1	1.4	221	193
Silicas 7	123	0.7	2.6	6.0	1.4	208	197
Silicas 9	146	1.1	2.3	5.8	1.4	182	195
Silicas 9	240	0.8	6.7	4.8	5.3	87	169
Silicas 10	322	0.3	6.9	6.0	5.3	Could not	172
						be
						determined
Silicas 11	204	0.7	6.4	5.7	5.4	101	173
Silicas 12	276	0.3	6.6	6.6	5.3	Could not	168
						be
						determined

APPLICATION EXAMPLES Example 1

For the investigation of the improvement in scratch resistance, a conventional 2-component polyurethane lacquer was used. The formulation of the lacquer and its production, including application, are summarised below:

Formulation:

	Parts
	by wt.

	Millbase
	Acrylic resin, 50% in xylene/ethylbenzene 3:1	53.3
	Butyl acetate 98%	6.7
	Xylene	6.7
	Silica	5.0
	Σ	71.7
	Lacquer make-up
	Acrylic resin, 50% in xylene/ethylbenzene 3:1	1.1
	Xylene	12.2
	Ethoxypropyl acetate	1.5
	Butyl glycol acetate	1.5
	Butyl acetate 98%	—
	Aliphatic polyisocyanate, approx. 75% in 1-	17.0
	methoxypropyl-2-acetate/xylene 1:1
	Σ	105.0

• Binder concentration: 40%
• Silica calculated on the basis of millbase (solids): 18.8%
• Silica calculated on the basis of lacquer (total): 5.0%
• Silica calculated on the basis of lacquer (solids): 12.5%
  Production and Application of Lacquers

The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, according to DIN ISO 1524. It must be smaller than 10 μm.

The conversion of the millbase to lacquer takes place in accordance with the formulation, the components being mixed at 2000 rpm with a vane agitator. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a quartz/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Sikron F500) and with a CaCO₃/water mixture (100 g water+1 g Marlon A 350, 0.25%+5 g Millicarb BG) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (200 irradiation angle).

TABLE 7
Summary of the properties of the liquid lacquers relevant in terms of lacquer technology,
and of the applied and dried films.

				Reference				Reference
	Comparative			without	Comparative		Silica	without
	silica 1	Silica 7	Silica 8	silica	silica 2	Silica 9	11	silica

Grindometer value [μm]	<10	<10	<10	<10	<10	<10	<10	/
Viscosity (millbase) [mPas]
6 Rpm	409	210	220	/	5670	935	832	/
60 Rpm	407	210	212	/	1260	409	407	/
Viscosity (lacquer + hardener)
[mPas]
6 rpm	120	80	80	60	446	195	175	55
60 rpm	113	82	82	61	194	146	144	64
Flow	poor	OK	OK	OK	Orange peel	OK	OK	OK
	fine cracks				effect

Scratch resistance

20° reflectometer value	81	89.5	89.1	91.3	38	85.5	85.3	91.7
before scratching
Haze before scratching	101	9	12	2	423	18	19	2
Black value My	272	286	286	291	260	283	282	294
40 strokes with Sikron	83.4	88.5	90.7	51.8	/	80.4	84.3	56.1
F 500 residual gloss [%]

The silicas 7+8 and 9+11 according to the invention can be used in high concentrations without impairing the appearance of the lacquer surface owing to their substantially lower rheological efficiency compared with comparative silica 1+2. In addition, the silicas according to the invention display a substantial improvement in scratch resistance of the lacquer surface.

Example 2

In this example the influence of the structural modification was investigated on the basis of a high solids 2-component PU clear lacquer. The formulation of the lacquer and its production, including application and testing, are summarised below:

Formulation:

	Parts by
	wt.

	Millbase
	Acrylic copolymer, mod. with synthetic	61.0
	fatty acids, 70% in n-butyl acetate
	Butyl acetate 98%	7.3
	Methoxypropyl acetate	1.7
	Solvesso 100	2.0
	Xylene	2.0
	Baysilon OL 17, 10% in xylene (silicone	0.7
	oil)
	Silica	5.0
	Σ	79.7
	Lacquer make-up (hardener)
	Aliphatic polyisocyanate, 90% in n-	22.3
	butyl acetate
	Butyl acetate 98%	2.0
	Solvesso 100	1.0
	Σ	105.0

• Binder concentration: 62.8%
• Silica calculated on the basis of millbase (solids): 11.7%
• Silica calculated on the basis of lacquer (total): 5.0%
• Silica calculated on the basis of lacquer (solids): 8.0%
  Production and Application of the Lacquers

The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, in accordance with DIN ISO 1524. It must be smaller than 10 μm.

The conversion of the millbase to lacquer takes place in accordance with the formulation, the components being mixed with a vane agitator at 2000 rpm. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a quartz/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Sikron F500) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (20° irradiation angle).

TABLE 8
Summary of the properties of the liquid lacquers
relevant in terms of lacquer technology, and of the applied
and dried films.

				Reference
	Comparative		Silica	without
	silica 1	silica 7	8	Silica

Bulk density [g/l]	50	146	123	/
Grindometer value [μm]	<10	<10	<10	/
Viscosity (millbase)
[mPas]
6 rpm	767	376	376	205
60 rpm	717	359	361	205
Viscosity (lacquer + hardener)
[mPas]
6 rpm	459	279	281	120
60 rpm	399	272	274	120
Flow	poor (fine	OK	OK	OK
	“cracks”)

Scratch resistance

20° reflectometer	82.3	86.5	86.3	88.2
value before
scratching
Haze before scratching	3	4	4	2
Black value My	275	283	282	292
40 strokes with	63.2	78.2	75.4	30.2
Sikron F 500
residual gloss [%]

The silicas 7+8 according to the invention can be used in high concentrations without impairing the appearance of the lacquer surface owing to their substantially lower Theological efficiency compared with comparative silica 1. In addition, the silicas according to the invention display a substantial improvement in the scratch resistance of the lacquer surface.

Example 3

For the investigation of the improvement of the scratch resistance, a conventional 2-component polyurethane lacquer was used. The formulation of the lacquer and its production, including its application, are summarised below:

Formulation

		Parts by wt.

	Millbase
	Acrylic copolymer, mod. with	43.4
	synthetic fatty acids, 60% solution
	Butyl acetate 98%	17.8
	Xylene	3.9
	Silica	5.0
	Σ	70.7
	Lacquer make-up
	Xylene	11.3
	Ethoxypropyl acetate	3.4
	Butyl glycol acetate	1.6
	Aliphatic polyisocyanate, approx.	18.6
	75% in 1-methoxypropyl-2-
	acetate/xylene 1:1
	Σ	105.0

• Binder concentration: 40%
• Silica calculated on the basis of millbase (solids): 19.2%
• Silica calculated on the basis of lacquer (total): 5.0%
• Silica calculated on the basis of lacquer (solids): 12.5%
  Production and Application of the Lacquers

The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, in accordance with DIN ISO 1524. It must be smaller than 10 μm.

The conversion of the millbase to lacquer takes place in accordance with the formulation, the components being mixed with a vane agitator at 2000 rpm. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a quartz/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Sikron F500) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (20° irradiation angle).

TABLE 9
Summary of the properties of the liquid lacquers relevant in terms of lacquer technology,
and of the applied and dried films.

				Reference				Reference
	Comparative			without	Comparative		Silica	without
	silica 1	Silica 7	Silica 8	silica	silica 2	Silica 9	11	silica

Grindometer value [μm]	<10	<10	<10	/	<10	<10	<10	/
Viscosity (millbase)
[mPas]
6 rpm	409	210	220	/	5670	935	832	/
60 rpm	407	210	212	/	1260	409	407	/
Viscosity (lacquer + hardener)
[mPas]
6 rpm	120	80	80	60	446	195	175	55
60 rpm	113	82	82	61	194	146	144	64
Flow	Poor	OK	OK	OK	Orange-peel	OK	OK	OK
	fine cracks				effect

Scratch resistance

20° reflectometer	81	89.5	89.1	91.3	38	85.5	85.3	91.7
value before
scratching
Haze before scratching	101	9	12	2	423	18	19	2
40 strokes with Sikron	83.4	88.5	90.7	51.8	/	80.4	84.3	56.1
F 500
Residual gloss [%]

The silicas 7+8 and 9+10 according to the invention can be used in high concentrations without impairing the appearance of the lacquer surface owing to their substantially lower rheological efficiency compared with comparative silica 1 and 2. In addition, the silicas according to the invention display a substantial improvement in the scratch resistance of the lacquer surface.

Example 4

Direct comparison of the silicas according to the invention with a scratch-resistant lacquer according to DE 198 11 790 A1, in which AEROSIL R 972 is used to improve the scratch resistance.

		Silicas 2)
	Prior art	according to the
	1)	invention

	Millbase
	Desmophen A 2009/1		190.2
	Methoxypropyl acetate:Solvesso		36.8
	100 1:1
	Silica		23.0
	Σ		250.0
	Lacquer make-up
	Desmophen A YEP4-55A,	96.0	—
	contains AEROSIL R 972
	Millbase	—	48.9
	Desmophen 2009/1	—	24.9
	OL 17, 10% in MPA	—	—
	Modaflow 1% in MPA	—	—
	MPA:Solvesso 100	11.6	33.8
	1:1
	Butyl glycol acetate	10.5	10.5
	Byketol OK	7.5	7.5
	Byk 141	0.8	0.8
	Addition of hardener
	Desmodur N 3390	23.6	23.6
	Σ	150.0	150.0

Production and Application of the Lacquers

• • 1) Comparative silica 1 is incorporated into the binder in accordance with DE 198 11 790 A1 using a jet disperser.
  • 2) The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, according to DIN ISO 1524. It must be smaller than 10 μm.

The conversion to lacquer of the millbases corresponding to 1) or 2) takes place in accordance with the formulation, the components being mixed at 2000 rpm with a vane agitator. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a CaCO₃/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Millicarb CaCO₃) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (20° irradiation angle).

TABLE 10
Summary of the properties of the liquid lacquers
relevant in terms of lacquer technology, and of the applied
and dried films.

		Prior art	Silica 7	Reference

	Grindometer value	<10	<10	/
	[μm]
	Viscosity (millbase)
	[mPas]
	6 rpm	58	30	30
	60 rpm	48	43	40
	Surface	Orange	OK	OK
		peel
	20° reflectometer	88.0	86.5	98.5
	value before
	scratching
	100 strokes with	88.6	96.3	59.6
	Millicarb
	Residual gloss [%]

It is shown that a substantially better improvement in the residual gloss is achieved after a scratch stressing of the lacquer surface by using the silica according to the invention than with the prior art. In addition, owing to its low rheological efficiency, the silica according to the invention does not cause an orange-peel effect.